1. Field of the Invention
The present invention relates to a digital modulator and demodulator circuit for modulating a fixed-length code into a variable-length code and demodulating a variable-length code into a fixed-length code.
2. Description of the Prior Art
In a conventional modulator circuit for modulating a fixed-length code into a variable-length code, as shown in FIG. 7, a serial fixed-length code (binary code) consisting of serially arranged bits is modulated into a serial variable-length code, for example, Run Length Limited Code (referred to as "RLL (2, 7) code " hereinafter) as disclosed in the Japanese Patent Publication No. 55-26494.
In more detail, input serial data is modulated in such a manner that the input data is entered to a shift register in synchronization with an input clock pulse signal where the shift register consists of a plurality of flip-flops serially connected, and then each output data of the shift register is decoded thereby to obtain an output serial variable-length code. In regard of the RLL (2, 7) modulation code used in the above case, the resulting post-modulation data is doubled in amount with respect to the pre-modulation data, and therefore the shift register is made to operate at leading and trailing edges of a input period of a clock pulse signal input in synchronization with the data.
Also, in a conventional demodulator circuit for demodulating a variable-length code into a fixed-length code, as shown in FIG. 8, a serial variable-length code (RLL (2, 7) modulation code) is demodulated into a serial fixed-length code (binary code) as disclosed in the Japanese Patent Publication No. SHO 55-26494. In more detail, input Serial data is modulated in such a manner that the input serial data is entered to a shift register consisting of a plurality of flip-flops serially connected to each other and then each output data of the shift register is decoded thereby to obtain an output serial fixed-length code.
However, in the conventional modulator circuit for modulating a fixed-length code into a variable-length code as mentioned above, since the input serial data is directly processed in the shift register, the modulation circuit is required to operate at a speed two times higher than the input timing of the input clock pulse.
Meanwhile, in the conventional demodulator circuit for demodulating a variable-length code into a fixed-length code as mentioned above, since the input serial data is directly processed in the shift register, the demodulator circuit is required to operate at a speed identical to the input timing of the input clock pulse.
The above-mentioned type of a modulator and demodulator circuit requires a high-speed device for processing data at a high speed such as an ECL (Emitter Coupled Logic) device in performing data modulation and demodulation at a high transfer rate to therefor require high-grade digital circuit techniques, which results in a technical difficulty in many cases.